The present invention relates to compositions for thermal energy storage or thermal energy generation, and more particularly, to a composition comprising a silica based gel or dry powder containing a water/urea phase change material for thermal energy storage or an endothermic or exothermic compound for thermal energy generation.
Phase change materials may be repeatedly converted between solid and liquid phases and utilize their latent heat of fusion to absorb, store and release heat or cool during such phase conversions.
These latent heats of fusion are greater than the sensible heat capacities of the materials. For example, in phase change materials, the amount of energy absorbed upon melting or released upon freezing is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material over an increment of 10.degree. C.
Upon melting and freezing, per unit weight, a phase change material absorbs and releases substantially more energy than a sensible heat storage material that is heated or cooled over the same temperature range. In contrast to a sensible heat storage material that absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point.
Ice/water phase change materials are low-cost, widely-used phase change materials for temperature regulation at 0.degree. C. Such phase change materials have found use in applications such as refrigeration, chilling of beverages, medical therapy, and frozen confections. The many applications for ice/water phase change material could be significantly increased if a means could be found to decrease the freezing temperature without a prohibitive concurrent decrease in thermal energy storage. Soluble additives such as salt, alcohol, glycol, glycerine, or sugar, all function to depress the freezing point of water to temperatures well below 0.degree. C. but these additives also decrease the heat of fusion to 50% or less that of pure water. Further, such additives, when mixed with ice/water, are messy and inconvenient to use.
If, for example, ice/water could be made to freeze and melt congruently at a temperature in the range of -11.degree. to -15.degree. C., without a substantial decrease in the heat of fusion and crystallization, many new applications would become possible and current applications improved. The new modified ice/water could be used to freeze pure water, make ice cream, keep cold drinks colder, store cool for off-peak electrical air conditioning, and thermal energy storage of "cool" for diurnal and seasonal cooling. Thus it could be used in a device for making frozen confections such as that disclosed in Uesaka, U.S. Pat. No. 4,488,817. Uesaka discloses using a cold-keeping agent which comprises water or carbonated water with organic or inorganic salts added thereto in a double-walled vessel for that purpose.
Further, if there were a means for containing the new modified ice/water so as to encapsulate it or to render it a dry powder, then, its use could be greatly expanded beyond that discussed above. In my U.S. Pat. Nos. 5,106,520 and 5,282,994 there is disclosed a free flowing, conformable powder-like mix of silica particles and a phase change material which may include water. Still there is no disclosure of the use of a water/urea clathrate or inclusion compound as the phase change material.
Thus, while phase change materials for thermal energy storage are known, improved thermal energy storage materials would be desirable. Likewise, improved endothermic and exothermic compounds for thermal energy generation are also in demand.
Instant cold and instant hot products for medical therapy and other uses are known. There are a number of instant cold packs on the market based on ammonium nitrate/water. These products usually contain a freezing point depressant (to prevent freezing to a hard ice in reuse); and, in some cases, a gelling agent as well to produce a somewhat reusable gel. One of the instant hot products commercially available is based on supercooling of a salt hydrate, sodium acetate trihydrate, that is initiated by mechanical attrition to start crystallization and supply hot at about 50.degree. C. The product can be reactivated by remelting in boiling water to provide limited reuse capability as a gel. With both the instant cold and instant hot products, the reusable gel has only a limited effectiveness and a limited useful life. Accordingly, a gel with a superior reuse capability would be advantageous. Perhaps even more advantageous would be a dry powder containing an endothermic or exothermic compound because of its soft conformability. As discussed above, dry powders containing phase change materials are disclosed in my U.S. Pat. Nos. 5,106,520 and 5,282,994, but there is no disclosure of using endothermic or exothermic compounds in that regard.
Accordingly, there is still a need in the art for improved compositions useful in thermal energy storage or thermal energy generation which are inexpensive and easy to use.